A common reaction for forming the octachlorodirhenate(III) anion is to heat tert-n-butylammonium perrhenate in benzoyl chloride at 209 ºC. If you look up the info for benzoyl chloride, one sees that the boiling point is only 197 ºC. To increase the boiling point, publications suggest closing off the system with a mercury bubbler, which increases the pressure enough to bump the boiling point of the liquid to 209 ºC.
In the teaching lab, use of mercury is highly frowned upon… are there any reasonable alternatives to a mercury bubbler to increase the boiling point (and thus, the reaction temperature) for this reaction? Publications report a 30-40% decrease in yield if the reaction temperature is lowered to 196 ºC, so there's a lot of motivation to keep the temperature up.